REPRODUCTION
LS Chapter 4
2 Types of Reproduction
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Sexual and Asexual
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“A”=without
• Asexual means without sex
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Advantages and disadvantages to each
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Some organisms can do either or both, depending on circumstances.
• Jellyfish
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Males release sperm, females release egg into the water
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They meet and fertilize and young jellyfish form on the sea floor
The young jellyfish then can reproduce asexually before becoming adults
Sexual Reproduction
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Type of reproduction that requires two organisms
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DNA from each of the parents come together and form unique offspring
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The female sex cell is the egg
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The male sex cell is the sperm
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The process of them coming together is called fertilization
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The fertilized egg is called a zygote
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The zygote then goes through the cell cycle and becomes multicellular
Chromosomes
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In interphase, DNA is in the form of chromatin
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It winds around proteins into a tight structure called a chromatid
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There are 2 of them, called sister chromatids
Sister chromatids attach at the centromere and form a chromosome (X-shaped)
Humans have 23 pairs of chromosomes (total of 46 chromosomes)
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1 from each pair come from mother, 1 from father
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I.E. 23 from mom, 23 from dad=23 pairs=46 chromosomes
The pair of chromosomes (1 from mom, 1 from dad) are called homologous chromosomes
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Homologous chromosomes have the same DNA, but may have different forms of a gene
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Example: One may have the gene for green eyes, the other may have the gene for brown eyes (much more to
come on this in the genetics section)
Meiosis
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Occurs in sexually reproducing organisms
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Splits cells into ½ the number of chromosomes
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Important!
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What if cells didn’t reduce their chromosome numbers?
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46 chromosomes from mom, 46 chromosomes from dad=92 chromosomes
• 2x as many as you should have
Very, very, very similar to mitosis…
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Same words are used, steps are the same
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Happens twice
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Stages: Interphase, Meiosis I, Cytokinesis, Meiosis II, another cytokinesis
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While mitosis produces 2 cells, meiosis produces 4 cells
Interphase
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Exactly the same as in mitosis
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Longest phase
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Period of growth and development
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Same stages
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G1 Phase-Growth
S Phase-DNA duplicates
G2 Phase-Growth and duplication of organelles
Meiosis
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Occurs in 2 phases
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Meiosis I
Prophase I
• Metaphase I
• Anaphase I
• Telophase I
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Cytokinesis
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Meiosis II
Prophase II
• Metaphase II
• Anaphase II
• Telophase II
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Cytokinesis
Prophase I
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Chromatin condenses into chromosomes (sister chromatids attached at the
centromere)
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Homologous chromosomes (1 from mom and 1 from dad) attach and exchange genes
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The nucleus breaks down
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Centrioles move towards poles of cell
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Spindle forms
Metaphase 1
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Spindle fibers attach to chromosomes at the centromere
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Homologous chromosomes line up in the equator of the cell
Anaphase I
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Homologous chromosomes are pulled apart
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1 chromosome moves to each end of the cell
Telophase I
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2 nuclei form
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1 around each set of chromosomes
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Spindle breaks down
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Chromosomes unwind into chromatin
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Cell begins to split
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Cytokinesis occurs
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The new cells enter a short resting phase before Meiosis II begins
Prophase II
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Meiosis II happens in both cells from meiosis I at the same time
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Chromatin winds up into chromosomes (sister chromatids attached at the
centromere)
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Nucleus breaks down
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Spindle forms
Metaphase II
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Spindle attaches to chromosomes
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Chromosomes line up in the center of the cell
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This time there is only 1, so the chromosomes are single file
Anaphase II
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Sister chromatids are pulled apart at the centromere
Telophase II
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4 nuclei form
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Spindles break down
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Chromatids unwind into chromatin
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Cells begin to divide
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Another cytokinesis
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4 cells are produced
Results of Meiosis
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Recall that in Prophase I, the homologous chromosomes exchanged genes
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This causes there to be 4 different cells produced from Meiosis
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Contrast this with the 2 identical cells resulting from mitosis
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Each of the cells produced from meiosis have ½ of the DNA as the parent cell
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In males, meiosis results in sperm cells
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In females, meiosis results in egg cells
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When the 2 cells fuse during fertilization, the DNA combines
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½ + ½ = 1 total cell
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This cell then undergoes mitosis
Advantages and disadvantages of Sexual
Reproduction
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Advantages:
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The offspring are unique due to meiosis
Except for twins, no 2 sexually-reproducing organisms have the same DNA
• The genetic variation makes organisms more likely to adapt and evolve to changing
environmental conditions
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Disadvantages:
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Slow
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Humans take 9 months, some elephants can be 2 years!
Requires energy
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Need to find a mate
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Some organisms use a lot of energy finding a mate
Asexual Reproduction
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Reproducing involving only one organism
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The offspring is a clone of the parent
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It is genetically identical to the parent
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All offspring are also identical to one another
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Some asexually-reproducing organisms can exchange parts of DNA to increase genetic diversity a little
4 main types:
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Fission
Budding
Regeneration
Vegetative Propagation
Fission
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Many prokaryotic organisms reproduce this way
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One organism splits into 2
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Similar to mitosis
Budding
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Offspring grows on the parent organism
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Occurs in yeast, hydras, and potatoes (though may be considered to be vegetative
propagation…depends who you ask)
Regeneration
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Offspring grows from a piece of the parent organism
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Happens in planarians, sea stars, and hydras, and other animals
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And Time Lords
Vegetative Propagation
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Similar to regeneration
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Only for plants
Offspring grow from part of the parent plant
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Strawberries, potatoes, spider plants
Advantages and Disadvantages of
Asexual Reproduction
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Advantages:
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Don’t need a mate
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Don’t need a lot of energy finding a mate
Quick
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Can produce many offspring quickly
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Some bacteria can split every few minutes!
Disadvantages:
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Low genetic diversity
Prone to extinction in changing conditions
• Unable to adapt for the same reason.
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